The present invention relates to a double-sided printing system for continuous forms and, more particularly, to a double-sided printing system for continuous forms for printing on both sides of continuous forms by sequentially operating two printers and a double-sided printing system for continuous forms for printing on both sides of continuous forms by sequentially operating two printing mechanisms which are provided within the system.
A printing apparatus using continuous forms as a printing medium is advantageous in that since the paper is fed by tractors provided with pins which engage the holes formed along both side edges of a form, no paper jam is caused during feeding and in that the paper is easily accommodated in a stacker.
If such continuous forms are printed on both sides, the paper can be very efficiently used. For this reason, printing apparatuses for printing on both sides of continuous forms have conventionally been proposed (e.g., Japanese Patent Laid-Open No. 99585/1981).
FIG. 39 shows the structure of the above-described conventional double-sided printing apparatus. In FIG. 39, the reference numeral 1 represents a printing controller and 2 printing portion. In the printing controller 1, the reference numeral 1a represents an interface portion for receiving printing data (top-surface printing data and back-surface printing data) such as character codes which are to be printed on both sides of continuous forms CF, from a host apparatus, 1b an converting portion for converting the top-surface printing data and back-surface printing data into dot printing image data for each page, 1c a switch, 1d and 1e first and second memories for storing printing image data which are to be printed on the top surface and the back surface, respectively, of each page, 1f and 1g reading portions for reading the printing image data from the first and second memories, respectively, and outputting the read data to first and second image forming portions 3a and 3b.
In the printing portion 2, the reference numerals 3a, 3b denote electrophotography type image forming portions, 4 a hopper for supplying the continuous forms CF, 5 a stacker for accommodating the printed continuous forms CF in a folded state, 6a, 6b tractors for feeding the continuous forms CF while engaging the holes formed along both side edges of the continuous forms CF with the pins provided on the outer peripheries of the tractors, 7a, 7b heat fixing rollers, and 8 paper feed rollers. In the image forming portions 3a, 3b, the reference numeral 3.sub.1 represents a photosensitive drum which has a photoconductive material (photosensitive material) on the surface and which rotates at a constant rate in the direction indicated by the arrow A. The radius of the photosensitive drum 3.sub.1 of the image forming portion 3b is much larger than the radius of the photosensitive drum 3.sub.1 of the image forming portion 3a. This is in order to enable the second image forming portion 3b to print on the back surface of the paper which is printed by the first image forming portion 3a. More specifically, the radius of the photosensitive drum 3.sub.1 of the second image forming portion 3b is determined so as to make it possible that when the back surface of the continuous forms fed after the top surface is printed by the first image forming portion 3a reaches the printing position of the second image forming portion 3b, the toner image produced on the photosensitive drum 3.sub.1 of the second image forming portion 3b reaches exactly the printing position.
The reference numeral 3.sub.2 denotes a pre-electrifier for uniformly electrifying the surface of the photosensitive drum 3.sub.1, and 3.sub.3 an electrostatic image forming portion for forming an electrostatic image on the photosensitive drum 3.sub.1. The electrostatic image forming portion 3.sub.3 has a pin electrode driving circuit 3.sub.31 and a pin electrode 3.sub.32. A multiplicity (corresponding to the width of the paper) of the pin electrodes 3.sub.32 are provided in the longitudinal direction of the photosensitive drum 3.sub.1. The reference numeral 3.sub.4 represents a developing portion for developing an electrostatic image into a toner image, 3.sub.5 a transferring electrifier for transferring the toner image to a continuous form CF, and 3.sub.6 a cleaner for eliminating the toner remaining on the photosensitive drum 3.sub.1 and cleaning the photosensitive drum 3.sub.1. The position of the transferring electrifier 3.sub.5 is the printing position.
When a negative voltage is applied by the pin electrodes 3.sub.32 to the surface of the photosensitive drum 3.sub.1 which has been uniformly positively, for example, electrified by the pre-electrifier 3.sub.2, the charges at that position are eliminated. Therefore, when a negative voltage is applied to the pin electrodes 3.sub.32 of the first and second image forming portions 3a, 3b on the basis of the dot image data, and the photosensitive drums 3.sub.1 are rotated, the latent images corresponding to the image to be printed are formed on the surfaces of the photosensitive drums 3.sub.1. Thereafter, the developing portions 3.sub.4 apply positively charged toner to the surfaces of the photosensitive drums 3.sub.1, so that the toner moves onto the electrostatic latent images. When the transferring electrifiers 3.sub.5 conduct corona discharge on the back surface of the continuous forms CF at a potential of the opposite (negative) polarity to that of the electrified toner images, the toner images are transferred to the continuous forms CF. The continuous forms CF with the toner images transferred by the transferring electrifiers 3.sub.5 are carried to the fixing rollers 7a, 7b so as to be heat-fixed.
The continuous forms CF are stored in the hopper 4 in a folded state, and they are fed from the hopper 4 in the direction indicated by the arrow B and stored in the stacker 5 in a folded state with the rotation of the tractors 6a, 6b. The printing controller 1 simultaneously inputs the printing image data for the top surface and the back surface so as to simultaneously form the electrostatic latent images on the photosensitive drums 3.sub.1. Since the radii of the photosensitive drums 3.sub.1 of the first and second image forming portions 3a, 3b are different, the transferring electrifier 3.sub.5 of the first image forming portion 3a first prints on the back surface of the continuous form CF, and the transferring electrifier 3.sub.5 of the second image forming portion 3b then prints on the top surface of the continuous form CF when the continuous form CF is carried to the transferring electrifier 3.sub.5 and the printing position thereof reaches the transferring electrifier 3.sub.5. This operation is repeated henceforth, thereby printing continuous forms CF on both sides of the continuous forms CF.
For the purpose of such double-sided printing, it is conventionally necessary to newly purchase an expensive double-sided printing apparatus even if there are already a plurality of single-side printing apparatuses. Accordingly, if it is possible to construct a double-sided printing system by using two single-side printing apparatus, it is advantageous in points of cost and space. Such a double-sided printing system can conveniently be used as two single-side printing apparatuses and as one double-sided printing apparatus as occasion demands.
In addition, in the above-described conventional double-sided printing apparatus, it is necessary to make the diameter of the photosensitive drum 3.sub.1 of the second image forming portion 3b much larger than that of the photosensitive drum 3.sub.1 of the first image forming portion 3a, which results in an increase in the size and the cost of the printing apparatus.